Clutch for a winch or hoist

ABSTRACT

A clutch comprises an input shaft, an output shaft and a biasing member. The biasing member is arranged to provide a biasing force that allows torque transfer from the input shaft to the output shaft via one or more input friction plates and one or more output friction plates. Rotation of the output shaft may cause the biasing force to vary to adjust the maximum torque setting of the clutch during reeling-in or reeling-out of a winch cable around a winch drum. A retainer may be included in the clutch which retains the biasing member in operative connection with the output shaft and translates axially relative to the input shaft upon rotation of the output shaft. The axial translation may cause the biasing force to vary by varying the compression of the biasing member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/715,962, filed May 19, 2015, the disclosure of which is incorporatedby reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a clutch for a winch, a winch systemand a method of adjusting a clutch torque setting.

BACKGROUND

When using a winch (or hoist) to raise a payload, a winch cable isattached to a motor-driven drum at one end and the payload at the other.The motor is driven to rotate the drum in order to gather up the winchcable on the outer circumference of the drum. As the cable is laid ontothe drum, the drum radius is effectively increased, due to the extraradius provided by the build-up of layers of cable laid thereon. Theincreasing radius increases the torque necessary to lift the payload,due to moment effects. In a modern hoist or winch system, a clutch isprovided to limit the torque applied to the drum. Unfortunately, in theevent of the aforementioned effective drum radius increase, the extratorque required must be somehow accommodated by the clutch, whichtraditionally only has a fixed setting (i.e. a maximum torque before itslips), set by the friction between the clutch plates. As the cable isreeled-in with a fixed line load, the torque generated may be too great,causing the clutch to slip. This could lead to the drum free-wheelingand the dropping of the payload.

SUMMARY

It is the aim of the present disclosure to overcome the aforementioneddisadvantages by providing a clutch for a winch that allows the clutchsetting to be varied as the winch cable winds up or down (is reeled-inor out), to account for the torque variation due to the variation ineffective drum radius. An auxiliary system to moderate for torquevariation due to friction-based variations may also be used in theclutch.

It will be appreciated by one skilled in the art that “winch” or “hoist”refer to the same apparatus, and for the purposes of this disclosure,the terms may be used interchangeably. Only a winch shall be referred toin the description, however, in accordance with the above, any referenceto a winch could equally refer to a hoist.

From one aspect, the present disclosure provides a clutch for driving awinch. The clutch comprises an input shaft, an output shaft and abiasing member. The input shaft is configured for connection to a motorand is operatively connected to at least one input friction plate forrotation therewith. The output shaft is configured for driving a winchand is operatively connected to at least one output friction plate forrotation therewith. The biasing member is arranged to provide a biasingforce that pushes the at least one input friction plate and the at leastone output friction plate into contact with each other so that torquecan be transferred from the input shaft to the output shaft via theinput and output friction plates. Rotation of the output shaft causesthe biasing force to vary.

In accordance with an embodiment of this aspect, the clutch may furthercomprise a retainer for retaining the biasing member in operativeconnection with the output shaft. Rotation of the output shaft causesthe retainer to translate axially relative to the input shaft. The axialtranslation of the retainer causes the biasing force to vary.

From another aspect, the present disclosure provides another clutch fordriving a winch. The clutch comprises an input shaft, an output shaft, abiasing member and a retainer. The input shaft is configured forconnection to a motor and is operatively connected to at least one inputfriction plate for rotation therewith. The output shaft is configuredfor driving a winch and is operatively connected to at least one outputfriction plate for rotation therewith. The biasing member is arranged toprovide a biasing force that pushes the at least one input frictionplate and the at least one output friction plate into contact with eachother so that torque can be transferred from the input shaft to theoutput shaft via the input and output friction plates. The retainer isconfigured to retain the biasing member in operative connection with theoutput shaft and to translate axially relative to the input shaft uponrotation of the output shaft. The axial translation of the retainercauses the biasing force to vary.

Reference to translation “axially” relative to a rotatable shaft shouldbe understood to refer to translation (i.e. movement) along a directionsubstantially parallel or aligned with the rotational axis of thatshaft.

The varying biasing force provided by the above aspects means that thefrictional engagement of the input and output friction plates variesand, as such, the torque setting at which the plates slip relative toeach other also varies. In use, the clutch may be configured so thatrotating the output shaft in the direction that causes a connected drumto gather a winch cable (and thus retrieve a load), causes the biasingforce to increase. The result of this is that the maximum torque thatcan be transmitted by the clutch (before the friction plates slip) isincreased. This may prevent the extra torque caused by the effectiveradius increase of the drum causing the clutch to prematurely slip. Inaddition, allowing the biasing force to decrease whilst the drum reelsout the winch cable (setting down a load) ensures that the clutch torquesetting decreases appropriately for the given load and effective drumradius. This ensures the clutch will slip before a harmful load isallowed to pass through the clutch and possibly be transmitted to thestructure to which the winch is attached.

In accordance with any of the above aspects or embodiments thereof, theretainer may comprise a retaining flange.

In accordance with any of the above aspects or embodiments, the biasingmember may comprise one or more disc springs positioned around theoutput shaft. For example, there may be three or more disc springs.

In accordance with any of the above aspects or embodiments, the axialtranslation of the retainer may cause the compression of the biasingmember or disc springs to be varied. Compressing the spring(s) mayincrease the biasing force provided.

Alternatively, other biasing members could be used such as anelastomeric block or a hydraulic arrangement.

In accordance with any of the above aspects or embodiments, the retainermay be operatively connected to the output shaft such that it isrotatable relative to the output shaft (i.e. the rotation of theretainer is not tied to the rotation of the output shaft).

The retainer may also be operatively connected to the output shaft suchthat it is axially translatable relative to the output shaft (i.e. itcan move axially relative to the output shaft).

The clutch may further comprise a toothed output flange extendingradially outward from the output shaft, a toothed portion extendingradially outward from the retainer, and an idler gear disposed radiallyoutward of the retainer and the output flange. The idler gear may be inspur-meshed engagement with both the output flange and the retainer.

The idler gear may have two separate engagement portions for engagementwith the output flange and the retainer, respectively. Alternatively, asingle engagement portion may engage both the toothed output flange andthe toothed portion of the retainer.

A gear ratio may be provided between the output flange and the retainer.

The output flange and retainer may have a different number of teeth.

The retainer may further comprise a first threaded portion on an innerradial surface thereof and the output shaft may further comprise asecond threaded portion on an outer radial surface thereof The first andsecond threaded portions may be co-operatively engaged. The secondthreaded portion of the output shaft may act as a lead screw to drivethe axial translation of the retainer relative to the output shaft.

Alternatively, the retainer may be operatively connected to the outputshaft such that it translates axially and rotates therewith (i.e. sothat it cannot rotate or move axially relative to the output shaft).

In this embodiment, the clutch may further comprise an auxiliary shaftarranged to be rotationally driven by the output shaft. Rotation of theauxiliary shaft relative to the output shaft may cause the output shaftto translate axially relative to the auxiliary shaft and the inputshaft.

The auxiliary shaft and the output shaft may be linked by a gearingarrangement that is configured to create the relative rotationtherebetween.

At least a portion of the auxiliary shaft may be positioned radiallyinside the output shaft.

The output shaft and the auxiliary shaft may co-operate via engagementbetween a threaded portion on a radially inner surface of the outputshaft and a threaded portion on a radially outer surface of theauxiliary shaft. The threaded portion of the auxiliary shaft may act asa lead screw to drive the axial translation of the output shaft relativeto the auxiliary shaft.

The clutch of any of the above described embodiments and aspects mayfurther comprise one or more ball-ramp assemblies positioned between theinput shaft and the at least one input friction plate. It should beunderstood that there may be intervening members between the ball-rampassembly or assemblies and that input shaft and the input frictionplate(s) respectively. In other words, the ball-ramp assembly is notnecessarily directly connected to the input shaft and the input frictionplate(s).

From a further aspect, the present disclosure provides a winch systemcomprising a clutch (as described in any of the preceding aspects orembodiments), a motor operatively connected to the input shaft and adrum operatively connected to the output shaft.

As discussed above, the winch system may further comprise a cablesecured to the drum. Rotating the output shaft in a first direction maycause the cable to be reeled-in around the drum and cause the biasingforce to be increased. Rotating the output shaft in a second directionmay cause the cable to be reeled-out from the drum and cause the biasingforce to be decreased.

The biasing force may be increased by rotation of the output shaft inthe first direction causing movement of the retainer and/or the outputshaft in a first axial direction. The biasing force may be decreased byrotation of the output shaft in the second direction causing movement ofthe retainer and/or the output shaft in a second axial direction.

From a yet further aspect, the present disclosure provides a method ofadjusting a clutch torque setting comprising the step of using theclutch of any of the embodiments and aspects described above. Thedisclosed method allows the clutch torque setting to be adjusted inorder to compensate for moment effects from an increase or decrease inthe effective drum radius.

It is to be understood that for the purposes of this disclosure“effective drum radius” is defined as the radius of a winch drumcombined with the width of the winch cable layers wound thereon. A winchcable layer is formed when the entire circumferential width of the drumhas been covered with winds of cable and the cable must be laid over thetop of the existing cable in order to be reeled on to the drum.

It is to be understood that for the purposes of this disclosure“spur-meshed engagement” describes the engagement of one gear withanother between respective toothed portions of the two gears.

It is to be understood that for the purposes of this disclosure, any“connections” between first and second parts may be direct or indirect,unless otherwise specified. An indirect connection between first andsecond parts may comprise one or more intervening members.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments and features of the present disclosure will now bedescribed by way of example only, and with reference to FIGS. 1 to 3 c,of which:

FIG. 1 is an overview of a winch system, in accordance with the presentdisclosure;

FIG. 2a is a cross-section of a clutch, in accordance with oneembodiment of the present disclosure, when the drum is at a minimumeffective radius;

FIG. 2b is an enlarged view of a portion of the clutch of FIG. 2 a;

FIG. 2c is an enlarged view of a portion of the clutch of FIG. 2a whenthe drum is at a maximum effective radius;

FIG. 3a is a cross-section of a clutch, in accordance with anotherembodiment of the present disclosure, when the drum is at a minimumeffective radius.

FIG. 3b is an enlarged view of a portion of the clutch of FIG. 3 a;

FIG. 3c is an enlarged view of a portion of the clutch of FIG. 3a whenthe drum is at a maximum effective radius;

DETAILED DESCRIPTION

FIG. 1 shows an overview of a typical winch system 1, as is known in theart. A driving means, such as a motor 2, is operatively connected to abrake 4 through gearing 3, which is operatively connected to a clutch 5,which is operatively connected to a drum 7 through gearing 6. The motor2 is activated to provide drive to the drum 7 through the gearing 3,brake 4, clutch 5 and gearing 6, in order to reel in (or out) a winchcable 7 a to lift (or drop) a payload 8. The drum 7 and cable 7 atogether form a winch 9. The brake 4 is provided as a means to stopcable winding without disengaging motor 1, whereas clutch 5 acts as amoderating means for the drive, allowing the drum 7 to be disconnectedfrom the motor 2 should the drum 7 become overloaded.

FIG. 2a shows a cross-section of one exemplary embodiment of a clutch 5in accordance with this disclosure. The clutch 5 is shown at thecondition when the drum 7 is at minimum effective radius (i.e. there isno cable wrapped around the drum). As shown, the clutch 5 comprises aninput shaft 10 connected to a drive shaft 9, which in use would beconnected to the motor 2 (not shown) and is rotatable about axis X. Theclutch 5 also comprises an output shaft 12 arranged co-axially with theinput shaft 10 and rotatable about axis X.

Bearings 56 a and 56 b support input shaft 10 and are attached tobracket 56, which holds the clutch 5 in place within the apparatus onwhich it is installed, such as a vehicle, for example an aircraft, suchas a helicopter (not shown). It should be understood, however, that thewinch could be used other than on a vehicle, for example on a crane(whether stationary or mobile) or on a building.

A seal 9 a is used to seal the connection between the drive shaft 9 andthe input shaft 10. The clutch 5 is used to selectively transfer therotation (drive) of the input shaft 10 to the output shaft 12.

Input friction plates 44 are operatively connected to the input shaft 10via a ball-ramp assembly 41, and intervening output friction plates 32attached to an output flange 34, which is attached to output shaft 12for rotating therewith. While this embodiment uses six input frictionplates and five output friction plates, any suitable number of eachplate could be used.

Although output flange 34 is rotatable with output shaft 12, it isslideable relative to output shaft 12 in the axial direction (i.e.parallel to axis X). Input friction plates 44 are attached to ball-rampflange 42 of ball-ramp assembly 41. Ball-ramp assembly 41 is operativelyconnected to input shaft 10 via balls 41 a, which are held in a recess41 c at one end of flange 42 and a recess 41 b in input shaft 10.

The clutch 5 further comprises a biasing member in the form of springpack 48 comprising three disc springs 48 a, 48 b, 48 c, also known asBelleville springs or washers. The spring pack 48 is connected to theoutput shaft 12 via spring pack retaining flange 50, such that thespring pack 48 and the spring pack retaining flange 50 rotate with theoutput shaft 12 (about axis X). The spring pack 48 is in mechanicalcommunication with a connecting arm 46 via a plurality of ball bearings42 a, so that it can rotate relative to the ball ramp flange 42. Bearing42 a is located in recesses in flange 42 and the connecting arm 46.

As will be understood by one skilled in the art, clutch 5 allowsrotation of the input shaft 10 to be transferred to the output shaft 12via spring loaded engagement of the input friction plates 44 with theoutput friction plates 32. The maximum permitted amount of torquetransferred from the input shaft 10 to the output shaft 12, and viceversa, can be controlled by adjusting the degree of spring loading. Thischanges the degree of frictional engagement between output frictionplates 32 and input friction plates 44.

The input shaft 10 is secured to an input housing piece 52. A seal isformed between the housing piece 52 and the input shaft 10, and thehousing piece 52 and the output shaft 12, by ring-seals 40 and 12 b,respectively. There is an additional ring-seal 12 a disposed between theoutput shaft 12 and input shaft 10, to ensure that the friction platecavity 30 is sealed from the surroundings. This is advantageous, as theclutch 5 of the present disclosure may be of a “wet-type” that isdesirably immersed in a fluid, such as oil. Such wet-type clutches haveadvantages such as better lubrication and/or heat management, as wouldbe apparent to one skilled in the art. It is to be understood, however,that the present disclosure is equally applicable to a “dry-type”clutch, and the use of a wet-type clutch in this description isexemplary only.

The function of ball-ramp assembly 41 will now be described in detail.While only two balls 41 a are shown, it should be understood that morethan 2 balls (and corresponding recesses) may be used.

It is known that during operation of a clutch, such as the describedclutch 5, the friction properties of friction plates 32, 44 can varygreatly during use and over the operational life of the clutch 5. Thisfriction property variation can cause unacceptable variations in themaximum permitted torque transfer between input shaft 10 and outputshaft 12, and may lead to the clutch slipping too easily or not easilyenough.

Ball-ramp assembly 41 is used to moderate and minimise the torquesetting variation encountered when operating the clutch 5, by adjustingthe force with which the input plates 44 are pressed upon output plates32. As described above, ball 41 a engages the input shaft 10 and theball-ramp flange 42 in opposing recesses 41 b, 41 c therein. As is knownin the art, such as disclosed in U.S. Pat. No. 3,511,349, the recessesare shaped to act as cam members that cams the ball 41 a to increase ordecrease the separation between the pairs of recesses 41 b, 41 c thathold each ball 41 a in place.

As the friction characteristics of the friction plates 32, 44 vary, theforce exerted by the input friction plates 44 on the output frictionplates 32 varies. If the coefficient of friction between the input andoutput friction plates 32, 44 increases, the maximum torque able to betransmitted through the clutch 5 before slipping will also increase.However, as the torque transmitted through the clutch 5 increases, theseparating force between the recesses (41 a, 41 b) will increase andthus, the friction between friction plates 32, 44 will decrease. Assuch, the maximum transmitted torque before slipping will subsequentlydecrease. As will be understood by one skilled in the art, the ball-rampassembly 41 therefore counteracts the effects of friction variations atthe friction plates 32, 44, and thus minimises them to retain the torqueslipping thresholds of the clutch 5. This provides an advantage overclutches not having a ball-ramp assembly.

The clutch 5 further comprises a centre (or ‘auxiliary’) shaft 11 thatis rotatably mounted in a co-axial arrangement radially within outputshaft 12 using a thrust bearing 20 and a bearing 21 therebetween. Thrustbearing 20 comprises bearing races 20 a, 20 b. Centre shaft 11 is fixedin axial position by the thrust bearing 20 and a locking nut 22. Aportion 13 a of the inner surface of output shaft 12 and a portion 13 bof the outer surface of the centre shaft 11 are co-operatively threadedto allow axial movement of the output shaft 12 relative to centre shaft11 when the shafts 11, 12 rotate relative to each other. The portions 13a, 13 b may comprise an Acme thread, or any other suitable thread style.The portions 13 a, 13 b are of a fixed length, to provide a fixedmaximum amount of axial translation, and the pitch of the threads (i.e.the distance between adjacent threads) is calibrated to provide adesired amount of axial movement per turn of the output shaft 12relative to centre shaft 11. Portion 13 b may act as a lead screw totranslate output shaft 12 axially. As shown, output shaft 12 and centreshaft 11 are operatively connected to clutch output gear assembly 14,comprising clutch output gear 15 and a smaller differential gear 16.Differential gear 16 is attached to output gear 15 such that both gears15, 16 rotate in unison. Output shaft 12 and centre shaft 11 haveexternal teeth 12 c, 11 a, formed on their free ends. The teeth 12 c, 11a engage output gear 15 and differential gear 16, respectively, by spurmeshing with teeth 15 a, 16 a on each respective gear 15, 16 (notshown). The clutch output gear assembly 14 allows output shaft 12 tooutput drive to the drum 7 to wind-up an attached winch cable 7 a, as isknown in the art. An additional function of the clutch output gearassembly 14 is to drive axial movement of the output shaft 12, relativeto centre shaft 11, by introducing a gear ratio between differentialgear 16 and output gear 15.

Differential gear 16 has a different number of teeth to the output gear15. In this specific example, differential gear 16 has fewer teeth thanoutput gear 15 which results in a gear reduction from output gear 15 todifferential gear 16, as will be understood by one skilled in the art.As such, when the clutch output gear assembly 14 is driven by clutch 5,the rotation speed of output shaft 12 will be greater than that ofcentre shaft 11. This rotational speed differential will cause arelative rotation between the two shafts, which in turn, causes outputshaft 12 to translate axially, as outlined above. The gear ratio can bechosen, along with the thread length and pitch of portions 13 a, 13 b,to provide the desired degree of axial translation per turn of shaft 11,as would be understood by one skilled in the art.

It is to be understood that this embodiment is exemplary only, anddifferential gear 16 could equally be provided with more teeth thanoutput gear 15 to provide the desired gear ratio. Furthermore, thedesired gear ratio could be achieved by any other suitable method, aswould be understood by one skilled in the art, for instance, the numberof teeth on the shafts 11, 12 could be different to each other.

As explained above, the spring pack 48 is connected to the output shaft12 by the retaining flange 50, such that the spring pack 48 and outputshaft 12 rotate together. In addition, retaining flange 50 is connectedto output flange 12 such that it translates axially therewith too. Thus,when output shaft 12 is translated axially so is spring pack 48. Thistranslation will compress spring pack 48 against connecting arm 46,which in turn, will be pushed against flange 42. Pushing against flange42 causes input friction plates 44 to engage the output friction plates32 more tightly (the output friction plates 32 are mounted to moveaxially with output shaft 12). As the drum 7 winds-up, the gear assembly14 will cause a progressive increase in the axial translation of outputshaft 12. This progressively increases the engagement force betweeninput and output friction plates 32, 44, which accounts for the extratorque required to lift or pull-in the payload due to the increasingeffective radius of the drum, as discussed above. This prevents theclutch 5 from slipping prematurely when the cable 7 a is wound up.

For illustration purposes, a comparison of FIGS. 2b and 2c shows themaximum axial translation A of output shaft 12 available and itscorresponding effect on spring pack 48.

FIG. 3a shows a cross-section of another exemplary embodiment of aclutch 105 in accordance with this disclosure. Clutch 105 is also shownat the condition when the drum 7 is at minimum effective radius (i.e.there is no cable wrapped around the drum). Clutch 105 comprises aninput shaft 110 connected to motor 2 (not shown) and rotatable aboutaxis X. Clutch 105 also comprises an output shaft 112 arrangedco-axially with the input shaft 110 and rotatable about axis X. Outputshaft 112 is supported in place by a roller bearing 111 a disposedbetween its outer surface and an inner surface of input shaft 110. Theroller bearing allows input shaft 110 and output shaft 112 to rotaterelative to each other. Output shaft 112 further comprises an outputgear 113 at an output end thereof, in order to facilitate transfer ofdrive to the winch drum 7 (not shown).

Bearing 156 a supports the input shaft 110 and bearing 156 b supportsthe output shaft 112. Bearings 156 a, 156 b may be attached to a supportstructure (not shown) to hold the clutch 105 in place on the apparatuson which it is installed. As with the previous embodiment, it may beinstalled on a vehicle, for example an aircraft, such as a helicopter,or may be installed on a something other than a vehicle, for example acrane (whether stationary or mobile) or on a building.

The clutch 105 is used to selectively transfer the rotation (drive) ofthe input shaft 110 to the output shaft 112.

Input friction plates 144 are operatively connected to the input shaft110 via a ball-ramp assembly 141, and intervening output friction plates132 attached to an output flange 134, which is fixedly attached tooutput shaft 112 for rotating therewith.

Input friction plates 144 are attached to ball-ramp flange 142 ofball-ramp assembly 141. Ball-ramp assembly 141 is operatively connectedto input shaft 110 via balls 141 a, which are held in a recess 141 c atone end of flange 142 and a recess 141 b in input shaft 110.

In the illustrated embodiment, clutch 105 is a wet-type clutch andfurther comprises an inner shaft 170 fitted co-axially inside outputshaft 112. Inner shaft 170 is secured to the output shaft 112 via snapring 170 b and locking nut 170 c. Inner shaft 170 is further supportedby roller bearing 170 a disposed between the input shaft 110 and theouter surface of the chamber 170. Inner shaft 170 further comprises acentral bore 171 along a portion of the axial extent thereof anddelivery channels 172 a through a radial width thereof. Deliverychannels 172 a are in fluid communication with delivery channels 172 bin the output shaft 112. The delivery channels 172 a, 172 b allowdelivery of a fluid (e.g. hydraulic fluid, oil, lubricants and/orcoolants) to the friction plate region of the clutch for cooling and/orlubrication, as already discussed above in relation to wet-typeclutches. As with the previous embodiment, wet-type clutch 105 is onlyexemplary and clutch 105 may be instead be a dry-type clutch. If clutch105 were to be a dry-type clutch, inner shaft 170 and its associatedfeatures would simply be absent from the clutch 105.

Clutch 105 further comprises a spring pack 148 comprising disc springs(i.e. Belleville springs or washers). In this particular embodiment, thedisc springs are co-axial with the output shaft 112 and centred aroundthe output shaft 112. The spring pack 148 is held in place via a springpack retaining flange 150. Retaining flange 150 includes a thread 150 aon an inner surface thereof, which is configured to co-operativelyengage with a thread 112 a on the outer surface of output shaft 112. Thethreads maybe acme threads or any other suitable thread type. The springpack 148 and the spring pack retaining flange 150 rotate with the outputshaft 112 (about axis X). The spring pack 148 is in mechanicalcommunication with a connecting arm 146 and a plurality of ball bearings142 a, so that it can rotate relative to the ball ramp flange 142. Theconnecting arm is supported around output shaft 112 by a roller bearing111 b. Bearing 142 a is located in recesses in the flange 142 and theconnecting arm 146. Spring pack 148 is fixed to retaining flange 150,such that it exerts a spring bias on connecting arm 146.

As with the previously described embodiment, clutch 105 allows rotationof the input shaft 110 to be transferred to the output shaft 112 viaspring loaded engagement of the input friction plates 144 with theoutput friction plates 132, and the maximum permitted amount of torquetransferred from the input shaft 110 to the output shaft 112, and viceversa, can be controlled by adjusting the degree of spring loading.

As illustrated, clutch 105 includes a ball-ramp assembly, operable inthe same manner as the ball-ramp assembly discussed in relation to theprevious embodiment. It is to be understood, however, that clutch 105need not include a ball-ramp assembly. For instance, the input shaft mayfurther comprise an input plate flange that is operatively connected tothe input friction plates and is biased directly by the spring pack,rather than through a ball-ramp assembly, as illustrated.

In the previously described embodiment, axial translation of spring pack48 was achieved by differential rotation of threadably engaged co-axialshafts. In the present, alternative embodiment, axial translation ofspring pack 148 is achieved via a different method.

Output shaft 112 further comprises a toothed flange 112 b extendingradially outward from output shaft 112. In addition, spring packretaining flange 150 further comprises a toothed portion 150 b extendingradially outward therefrom. A toothed idler gear 160 is disposedradially outward of retaining flange 150 and output shaft flange 112 b.Idler gear 160 is comprised of a central supporting bolt 160 a andbearings 160 b, 160 c which support toothed engagement portions 165 and162. A washer 160 d is used to space the head of bolt 160 a frombearings 160 b, 160 c. The threaded end of bolt 160 a may be used toattach idler gear 160 to a supporting structure within the clutch 105 orwinch assembly (not shown). Engagement portions 165 and 162 areconfigured to be in spur-meshed engagement with the toothed portion ofthe spring pack retaining flange 150 b and the output shaft flange 112b, respectively. Engagement portions 165 and 162 may be part of the samegear or provided on separate gears.

As the winch cable 7 a is reeled in and drive is transmitted from motor2 through clutch 105, rotation of output shaft 112 and flange 112 b willrotate the idler gear 160 and transmit drive to the spring packretaining flange 150 via toothed portion 150 b. In a similar way to thedifferential gears of the previous embodiment, flange 112 b has adifferent number of teeth to that of portion 150 b. Therefore outputshaft 112 and spring pack retaining flange 150 will rotate at differentspeeds. The threaded engagement between threads 112 a and 150 a isconfigured such that differential rotation of output shaft 112 andspring pack retaining flange 150 causes the spring pack retaining flange150 to be translated axially towards connecting arm 146, via rotationaround thread 112 a (which acts as a lead screw). The axial translationof spring pack retaining flange 150 closer to connecting arm 146compresses the spring pack 148 and thus exerts greater spring bias onthe input friction plates 144. By exerting greater spring bias on theinput friction plates 144 the maximum torque tolerance of clutch 105 isincreased, which compensates for the increasing moment effect due to theincreasing effective drum radius during reeling in of cable 7 a, aspreviously discussed.

In this particular embodiment, engagement portions 162 and 165 haveidentical numbers of teeth and toothed portion 150 b has fewer teeththan flange 112 b. This introduces a reduction gear ratio between flange112 and portion 150 b. This allows retaining flange 150 to rotate aroundthread 112 a via the engagement of thread 150 a therewith. As will beappreciated by one skilled in the art, the thread pitch and differentialnumber of teeth between portion 150 b and flange 112 b can be calibratedto provide a desired amount of axial translation per rotation of outputshaft 112. This provides a progressive increase in spring bias to matchthe progressive increase in effective drum radius, as cable 7 a isreeled in. It will also provide a matching progressive decrease inspring bias to match the progressive decrease in effective drum radiusas cable 7 a is reeled out).

It should be appreciated that within the scope of this disclosure, agear ratio may be imparted by any suitable method, as would beunderstood by one skilled in the art. For instance, in anotherembodiment, flange 112 b and portion 150 b may have the same number ofteeth, and portions 162 and 165 of idler gear 160 may have differentnumbers of teeth to provide the gear ratio between portion 150 b andflange 112 b. In a further embodiment, portions 162 and 165 could have adifferent number of teeth and portion 150 b and flange 112 b could havea different number of teeth also.

For illustration purposes, a comparison of FIGS. 3b and 3c shows themaximum axial translation A of spring pack retaining flange 150available, and its corresponding effect on spring pack 148.

It is to be appreciated that the ball-ramp mechanism illustrated inFIGS. 2a and 3a is an optional feature, and these embodiments need notinclude a ball-ramp assembly at all. For instance, the input shaft mayfurther comprise an input plate flange that is operatively connected tothe input friction plates and is biased directly by the spring pack,rather than through a ball-ramp assembly, as illustrated. Such a clutchmay be less complex and less expensive than one including a ball-rampassembly.

The ball-ramp assembly provides a separate and additional mechanism tothe axial translation of the retaining means. The axial translation ofthe retaining means varies spring pack compression to compensate for thevariation in drum effective radius during reeling in/reeling out, whichis the primary objective of this invention. The ball-ramp mechanism maycompensate additionally for frictional variances within the clutch atthe friction plates. The combination of the two in the same clutch, suchas in the illustrated exemplary embodiments in FIGS. 2a and 3a , mayprovide a particularly advantageous clutch with further reduced torquesetting variation, compared to a clutch with an axial translationfeature alone. The ball-ramp mechanism is not, however, essential to thepresent disclosure.

Although the figures and the accompanying description describeparticular embodiments, it is to be understood that the scope of thisdisclosure is not to be limited to such specific embodiments, and is,instead, to be determined by the scope of the following claims.

The invention claimed is:
 1. A clutch for driving a winch, the clutchcomprising: an input shaft for connection to a motor and beingoperatively connected to at least one input friction plate for rotationtherewith; an output shaft for driving a winch and being operativelyconnected to at least one output friction plate for rotation therewith;a biasing member arranged to provide a biasing force that pushes the atleast one input friction plate and the at least one output frictionplate into contact with each other so that torque can be transferredfrom the input shaft to the output shaft via the input and outputfriction plates, wherein rotation of the output shaft causes the biasingforce to vary; a retainer for retaining the biasing member in operativeconnection with the output shaft, wherein rotation of the output shaftcauses the retainer to translate axially relative to the input shaft,the axial translation causing the biasing force to vary; wherein theretainer is operatively connected to the output shaft such that ittranslates axially and rotates therewith; and an auxiliary shaftarranged to be rotationally driven by the output shaft, wherein rotationof the auxiliary shaft relative to the output shaft causes the outputshaft to translate axially relative to the auxiliary shaft and the inputshaft.
 2. The clutch of claim 1, wherein the retainer comprises aretaining flange.
 3. The clutch of claim 1, wherein the biasing membercomprises one or more disc springs positioned around the output shaft.4. The clutch claim 1, wherein the axial translation of the retainercauses the compression of the biasing member to be varied.
 5. The clutchof claim 1, wherein the auxiliary shaft and the output shaft are linkedby a gearing arrangement that is configured to create the relativerotation therebetween.
 6. The clutch of claim 1, wherein at least aportion of the auxiliary shaft is positioned radially inside the outputshaft.
 7. The clutch of claim 6, wherein the output shaft and theauxiliary shaft co-operate via engagement between a threaded portion ona radially inner surface of the output shaft and a threaded portion on aradially outer surface of the auxiliary shaft, and the threaded portionof the auxiliary shaft acts as a lead screw to drive the axialtranslation of the output shaft relative to the auxiliary shaft.
 8. Awinch system, comprising: a clutch as claimed in claim 1; a motoroperatively connected to the input shaft; and a drum operativelyconnected to the output shaft.
 9. The winch system of claim 8, furthercomprising a cable secured to the drum, wherein rotating the outputshaft in a first direction causes the cable to be reeled-in around thedrum and causes the biasing force to be increased, and rotating theoutput shaft in a second direction causes the cable to be reeled-out offthe drum and causes the biasing force to be decreased.
 10. A method ofadjusting a clutch torque setting comprising the step of using theclutch of claim 1 to compensate for moment effects from an increase ordecrease in the effective drum radius.